The invention relates to a charge-coupled transversal filter comprising a charge-coupled device having a number of series-connected stages, in which there are present first means for supplying an input signal, second means by which a number of weighted signals are formed from this input signal by multiplying this input signal by weighting factors and by which the weighted signals are introduced in parallel into the said stages of the charge-coupled device, as well as third means by which the introduced signals are transported through the charge-coupled device to an output which is connected to reading means, the charge-coupled device being of the buried channel type and having a charge transport channel located at a surface of a semiconductor body and having the form of a surface layer of mainly the one conductivity type, which can be depleted throughout its thickness and is provided with a row of electrodes forming part of the said third means and comprising a group of surface zones of the second conductivity type, which are each associated with one of the said stages of the charge-coupled device, while there is provided in/on each of these surface zones a zone of the first conductivity type, which is separated from the semiconductor layer of the first conductivity type by an intermediate part of the associated surface zones of the second conductivity type.
Charge-coupled transversal filters are frequently described in the literature. Typically, a charge-coupled device (CCD) is used which is composed of MOS elements. In this device the electrodes serving for the storage and the transport of charge packets are provided in the form of conductive layers on a thin oxide layer above the charge transport channel. In a compact embodiment, the input signals are serially supplied to the input of the CCD and the weighted output signals are derived in parallel at the various stages of the CCD and are added to each other in a summation device.
A charge-coupled transversal filter of the kind desribed above is known inter alia from the article "A PISO JCCD Filter with High-speed Linear Charge Injection" by E. A. Wolsheimer, published in IEEE Journal of Solid State Circuits, Volume SC-18, No. 2, April 1983, p. 193/200. In this device a CCD of the buried channel type is used, the charge transport channel being formed in an n-type epitaxial layer which is grown on a p-type substrate. The electrodes are constituted by p-type surface zones which form pn junctions of the epitaxial layer. Because of these junctions this device is designated as a JCCD (Junction CCD). During operation, the junctions are biased in the reverse direction. In contrast with the conventional transversal filter described above, there are formed from each of the input signals a number of weighted signals which are supplied in parallel to the CCD, passed through the charge transport channel and added in the CCD to weighted signals already introduced. The added signals are transported through the channel to the output of the channel, at which the output signals can be derived in series. (PISO=Parallel In--Serial Out). This known device inter alia has the advantage that a bipolar technique can be used for its manufacture so that the filter can be integrated together with a bipolar circuit on a common semiconductor body. For the introduction of the weighted input signals, n-type zones can be formed in the associated p-type zones, as a result of which a bipolar transistor structure is obtained, in which the n-type zone constitutes the emitter, the p-type surface zone serving as a CCD electrode constitutes the base and the subjacent part of the n-type charge transport channel constitutes the collector. In the publication mentioned above, a method is described of introducing the weighted signals as emitter currents at the input stages of the CCD.
In this known filter, a voltage-to-current conversion is carried out for each input stage. Beside the filter coefficients or weighting factors formed outside the CCD, this method requires a fast peripheral electronic circuitry. Moreover, two JCCD lines are used, i.e. one for the positive weighting factors and a second for the negative weighting factors.